Most modern transmissions are equipped with pumps. Transmission pumps are generally driven by the engine crankshaft. Of the sort of pumps that are compatible with today's transmissions are fixed displacement pumps and variable displacement pumps. Fixed displacement pumps provide the same flow per revolution output regardless of engine speed. A variable displacement pump (or VDP) provides a variable flow rate output which depends on engine speed and transmission system flow requirements (which are commonly referred to as “flow demand”). While there are tradeoffs associated with using either variable displacement or fixed displacement pumps, variable displacement pumps can reduce the total power consumed by the transmission pump by delivering only the flow required by the transmission hydraulic system. For most variable displacement pumps at lower speeds pump loss is directly proportional to engine speed and then pump loss plateaus over a certain speed. In this way, variable displacement pumps offer greater powertrain efficiency.
A displacement decrease circuit can be incorporated in hydraulic circuits for variable displacement pumps in order to provide a command pressure signal to actuate the bore ring yielding a lower displacement at higher speeds. A regulator valve can be incorporated in the hydraulic circuit to selectively link a displacement decrease circuit to the bore ring. Phase lag between the regulator valve and pump bore ring can cause oscillations in both line and decrease circuits (for example as shown in FIG. 6). In the past a controlled leakage has been incorporated in the displacement decrease circuit to mitigate oscillations. However, hydraulic controlled leakages may result in significant flow losses (or hydraulic losses) up to 20% of total typical transmission hydraulic flow demand. The hydraulic loss is proportional to displacement decrease pressure and increases with engine speed when the system commands low displacement (or higher decrease pressure). This hydraulic loss results in an overall increase of engine power required to maintain hydraulic flow and leads to a reduction in fuel economy.
Therefore, it is desirable to have a more effective control circuit for a variable displacement pump that mitigates pressure oscillations in the control circuit when the pump regulator valve is regulating the pump at a displacement less than the pump maximum level. There also exists a need for a method of manufacturing a hydraulic control circuit for a vehicle transmission pump having the same utility.